My car started jerking when shifting from 3rd to 4th gear a few months ago. The transmission fluid has been flushed, and the transmission system has been re-lubricated everywhere that doesn't involve taking the entire transmission out. The problem isn't consistent. Sometimes it shifts just fine. The RPM's also don't rev past 3, even when it hesitates and jerks before shifting. Any idea what could be going on?
Since you said the problem only happens between 3rd and 4th gear, you likely have an internal transmission problem. An engine performance problem would likely be noticeable in all gears. Also, since the problem only happens when shifting from 3rd to 4th, a problem that affects the entire transmission, such as fluid level/condition, is probably not the problem. My guess is that there is a problem with the valve body or one of the internal transmission control solenoids. I suggest you have a trained professional inspect your vehicle first hand for an accurate fix to your shifting issue.
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